1. Field of the Invention
This invention relates to a zoom lens barrel assembly for a camera, and, more particularly, to a power driven zoom lens barrel assembly for a camera which is operated by a single power drive element to achieve both adjusting a zoom ratio of the zoom lens and focusing the zoom lens.
2. Description of Related Art
In most zoom lenses, lens groups of the zoom lens are moved along optical axis by means of a cam mechanism differently between focusing the zoom lens on a subject and adjusting a zoom ratio of the zoom lens. To enable the zoom lens to achieve focusing and zooming by means of a power drive mechanism, there have been proposed several techniques. Specifically, one of the techniques employs separate power drive elements such as electric motors for focusing and zoom ratio adjusting, respectively. Another technique employs a single power drive element. In the case where a single power drive element is used for both focusing and zoom ratio adjusting, there have been known three ways. One of them is to divide driving power of the single power drive element into two power transmission paths for a focusing mechanism and a zoom ratio adjusting mechanism. The second is, as known from Japanese Patent Publication No. 6-100707, to incorporate a mechanism common to focusing and zoom ratio adjusting and achieve focusing and zoom ratio adjusting alternately. The last one, as known from Japanese Unexamined Patent Publication No. 9-43487, incorporates a restraint mechanism which causes relative axial movement along the optical axis between two lens groups of the zoom lens to achieve zoom ratio adjusting and locking axial movement of one of the two lens groups relative to the other during focusing.
The utilization of two power drive elements always yields an increase in camera price. The single power drive element cooperating with separate drive mechanisms for focusing and zoom ratio adjusting always needs an increased number of parts with an adverse effects of not only increasing costs but also increasing an overall size of the camera. The combination of a single power drive element and a common drive mechanism has a necessity for providing alternate lens guide cam slots for focusing and zoom ratio adjusting which makes a lens drive mechanism quite complex. While the last zoom lens drive system can remove the above drawbacks, however, the locking mechanism incorporates an electromagnetic solenoid to disconnect cooperation of one of the two lens groups with its associated cam to lock the axial movement of the lens group and holds the lens group with friction only so as to remain immovable which possibly leads to unstable focusing operation.